Differences in the Kinetics of Dopamine Uptake in Synaptosome Preparations of the Median Eminence Relative to Other Dopaminergically Inervated Brain Regions

Abstract

A comparison of the kinetics of dopamine uptake was made in synaptosomal preparations of the median eminence, striatum and olfactory tubercle. Double reciprocal Lineweaver-Burk plots of the initial velocity versus the concentration of dopamine yielded a single straight line in all three areas. The Michaelis constant (K_m) in the median eminence (1.8 ± 0.9 × 10^–6 M) was significantly higher (p<0.05) than in the striatum (4.2 ± 0.8 × 10^–8 M) or olfactory tubercle (6.3 ± 1.9 × 10^–8 M). Uptake in the median eminence appeared to be predominantly into dopaminergic terminals since preincubation with desipramine did not affect the maximum velocity (V_max) of dopamine uptake. Observed uptake was predominantly due to transport across the neuronal membrane and not into storage granules, since reserpine only caused a small decrease in uptake. The low affinity of dopamine uptake in median eminence synaptosomes is consistent with the neurose-cretory nature of these terminals, whereas, in the striatum and olfactory tubercle, high affinity reuptake is consistent with the role of dopamine as a neurotransmitter.